As is well known in the art of pressure measurement, a pressure transducer typically consists of two general components: a component that responds mechanically to a change in pressure, i.e. by e.g. changing shape, and a component that senses the mechanical response of the other component and provides a signal that can be correlated with the mechanical response, i.e. a strain sensor.
For measuring pressure, such as pressure in a fluid, the mechanically responding component is often a cylindrical body. One way of measuring pressure is to sense how a cylindrical body will lengthen, in what is called Poisson's effect, in response to pressure imposing a radial stress on the body leading to circumferential stress, also called hoop stress. In Poisson's effect, when pressure, such as fluid pressure, squeezes radially on a cylindrical body, the body tends to lengthen as it thins, i.e. it experiences axial strain as well as circumferential strain.
The prior art also teaches that it is also useful to sense how a cylindrical body thins, instead of how it lengthens, in response to an increase in pressure acting on the cylindrical walls of the cylindrical body. Sensing either aspect of the mechanical response, either the lengthening or thinning response to an increase in pressure, can be done by the second component of a pressure transducer.
To measure axial strain of a cylindrical body exposed to some pressure, a fiber optic, having a Bragg grating, is often arranged along the length of the body and attached so as to lengthen along with the body. A Bragg grating is created over a length of a fiber optic by exposing segments along the length to different light in the ultraviolet range causing different indices of refraction. The axial strain is then detected by interferometry, i.e. when light is passed through the fiber, the Bragg grating causes an interference pattern that depends on the length over which the Bragg grating extends; when the length changes, as a result for example of fluid pressure and Poisson's effect, the pattern changes and does so in a way that allows the change in length to be determined, which can then be correlated with pressure that caused the change in length.
An alternate method of using an optical fiber Bragg grating as one component of a pressure transducer to sense how a cylindrical body, serving as the other component, strains axially in response to a change in pressure is to create a Bragg grating on either end of a length of optical fiber lengthening as a result of Poisson's effect. This method has a far greater sensitivity than the single Bragg grating approach, because a greater length of fiber is strained yielding a greater overall change in length.
When a body deforms in a way that exhibits Poisson's effect, so that strain parallel to the applied stress is accompanied by strain orthogonal to that stress (e.g. squeezing circumferentially on a cylindrical body not only makes it thinner but also makes it longer), the ratio of the orthogonal strain to the parallel strain is known as Poisson's ratio and is an indicator of the magnitude of Poisson's effect for the particular material or structure composing the body.
The use, as a temperature sensor, of an optical fiber with a Bragg grating is not new, and such use includes embedding an optical fiber sensor in a plurality of layers of resin reinforced by (non-optical) fibers. U.S. Pat. No. 5,399,854 to Dunphy et al. teaches embedding an optical fiber sensor in a plurality of layers of fiber-reinforced resin, each layer having a different thermal expansion coefficient, and the reinforcing fibers of each layer oriented to cause transverse stresses on the embedded optical fiber different from those caused by the other layers, the difference depending on the temperature being sensed. The unequal transverse stresses cause birefringence in the grating, which can be correlated with the temperature being sensed.
In contrast, according to the prior art, in order to make a pressure sensor sufficiently sensitive, bare (i.e. unsheathed) optical fiber is often used, so that the optical fiber having a Bragg grating is exposed to the full pressure, undiminished by any sheathing. But bare optical fibers are susceptible to abrasion and chemical attack, so that in some applications, using ensheathed optical fibers is not practical.
Sometimes in the prior art, bare optical fibers are sheathed in a fine diameter steel capillary tube filled with fluid to protect against chemical attack and abrasion. However, such a sheathing reduces the sensitivity of the optical fiber. Furthermore, steel tubing has a different coefficient of thermal expansion than optical fiber material, and this difference creates thermal-based axial strains that compound the pressure measurement. If one could assume that the optical fiber would expand with the steel capillary, one could subtract out the effect of the thermal strains. However, the optical fiber can slip within the metal capillary, so the thermally induced strains are difficult to predict and thus distinguish from pressure induced strain.
In other prior art, an optical fiber is made more sensitive to pressure by encapsulating or jacketing the optical fiber in a soft polymer having a relatively low bulk modulus of elasticity and a relatively high Young's modulus, and using a jacket outer diameter as large as 2000 microns on an optical fiber with a diameter of 125 microns. A disadvantage of these polymer coatings is their very high sensitivity to temperature changes due to the very large coefficient of thermal expansion of these polymers. Changes in temperature cause very large expansions of the polymer coatings; these expansions strain the optical fiber and the Bragg grating giving a false indication of a pressure change, and sometimes damaging the optical fiber.
Even the bare optical fiber itself will respond to temperature changes by undergoing thermal expansion or contraction in both length and diameter, but these changes in dimension can be compensated for by using a second grating that is not exposed to the pressure. However, with a bare optical fiber, even a flowing of fluid over the optical fiber can, through shear stresses, impart axial stresses that interfere in the pressure measurement.
What is needed is a mechanical form, for use as the mechanical component of a pressure transducer, that will not itself experience significant thermal strains, but will exhibit a pronounced Poisson's effect when exposed to a change in pressure acting on the mechanical form, and so exhibit significant axial and longitudinal strains. When used with an optical fiber having a Bragg grating as the second component of a pressure transducer, the mechanical form should not reduce the sensitivity of the optical fiber to the pressure being measured, even if it ensheathes the optical fiber and so protects the optical fiber against abrasion and chemical attack.
In some applications, a cylindrical body used as the mechanical component of a pressure sensor can extend over a distance spanning regions where sensitivity to pressure is not wanted, and other regions where it is. Because of this, an even more advantageous mechanical form would allow varying sensitivity to pressure along its length, so that it is more sensitive to pressure along some spans, and substantially insensitive along other spans.
Another important advantage would be for the mechanical form to be producible in a continuous batch process, so that there would be no break between lengths of the form intended to exhibit different sensitivities to pressure. In other words, ideally, the manufacturing process would produce, as the mechanical form, a continuous material, although differing in its construction in different spans, according to the level of response to pressure wanted by the different spans.